1. Field of the Invention
The present invention relates to a fuel cell, especially a separator for fuel cell.
2. Prior Art
A conventional PEFC (Polymer Electrolytic Fuel Cell) has a main feature in that it comprises a polymer membrane solid electrolyte, and carbon electrodes each carrying a catalyst such as platinum, and so on. The polymer electrolytic fuel cell is structured such that the carbon electrode is sandwiched between a pair of separators, the respective separators for forming flow channels for hydrogen gas as fuel, and an oxidizing agent gas (oxygen or air, and so on), respectively, and functioning as a current collector. This is called a single cell, and a fuel cell stack is made up by stacking the plural single cells one on top of another.
The separators among those constituent members are a member for efficiently supplying a reacting gas to the respective electrodes, and is made of a carbon-based or metal-based conductive material. In this case, the reacting gas is a generic term referring to a fuel gas as well as the oxidizing agent gas.
Besides, the separator is classified into several types depending on difference in the structure of a face thereof, in contact with the respective electrodes or a diffusion layer. There are available, for example, a separator whose face in contact with the electrode (diffusion layer) is uneven, and a separator made up by combining a flat sheet with an interconnector having a face in uneven shape, or a groove-like shape. The constituent material of the separator can be broadly classified into the carbon-based conductive material, and the metal-based conductive material. Widespread use of the metal-based conductive material is under study because it is low in material cost, and excellent in mass productivity. Further, since use is made of a thin metal sheet in this case, use of the metal-based conductive material has a merit in that a separator can be fabricated so as to be compact in size, and lighter in weight.
A carbon-based separator, in the main, and so on are relatively superior in formability for forming a manifold, and a flow channel groove, and have been adaptable to material having a thickness not less than on the order of 1 to 2 mm. In the case of a separator fabricated out of the thin metal sheet by press working, however, it has been difficult to apply the conventional technology thereto.
Further, a function required of the separator is to efficiently supply the reacting gas to the electrodes. If the constituent material of the separator is carbon-based, it is possible form flow channels in an optional shape, so that an efficient separator can be obtained with ease, however, if the constituent material of the separator is metal-based, flexibility in formability is low as compared with the case of the constituent material being carbon-based because of limitations of plastic working applied to metal. With a separator made of a graphite material, it is possible to form a serpentine structure (winding flow channel; a structure for enabling a flow rate of the reacting gas to increase, thereby evenly supplying the reacting gas to the face of the electrode) on both sides of one sheet of the separator. Meanwhile, in the case where an attempt is made to form an internal manifold type separator as a metal separator for the fuel cell out of a thin metal sheet, the thin metal sheet is worked on such that flow channel grooves, projections, and so on are formed at the central part of the thin metal sheet, and the periphery thereof, respectively, to form pseudo-serpentine flow channels, thereby shaping flow channels for altering a flow direction of the reacting gas.    Patent document 1: Japanese Patent Application Laid-open Publication No. 2005-166576